Positioning on one's side while sleeping, resting, or under anesthesia causes increased pressure on the shoulder and axillary structures (i.e., structures associated with an armpit) which, in turn, may cause injury to these structures, resulting in shoulder pain, arm pain, or radiating nerve pain. People with shoulder or neurologic disorders who try to sleep in a sidelying (lateral) position commonly awaken with shoulder pain and/or numbness or tingling of the hand or arm. Such symptoms may cause significantly altered sleep patterns and result in other health problems.